Crew from one of the world's fastest single hull racing yachts linked arms to survive a freak accident in the Fastnet race which left them stranded for hours in surging waves in the Celtic Sea.

Five sailors from Rambler 100, which was leading the race off the south coast of Ireland, were thrown from their vessel when its keel snapped off in rough seas and it "turned turtle just like a dinghy capsizing", according to Mick Harvey, the boat's project manager.

With some difficulty, the remaining 16 crew managed to clamber on to the upturned hull of the stricken vessel where they awaited rescue for two and a half hours in foggy conditions before dark on Monday evening.

The five who were thrown clear close to the turning point at Fastnet rock were swept out of sight and linked arms in a circle to survive. Among them was the boat's owner George David, 69, the former chairman of United Technologies Corporation, along with his girlfriend, Wendy Touton.

"Soon after rounding the Fastnet Rock, the wind went south-west right on the nose," Harvey said. "We were beating into big seas, launching the Rambler off the top of full size waves.

"I was down below with navigator, Peter Isler, when we heard the sickening sound of the keel breaking off.

"It was instantaneous – there was no time to react. The boat turned turtle, like a dinghy capsizing. Peter issued a Mayday and we got out as quickly as we could. It was a scary moment, one that I will never forget. I can't begin to tell you how relieved I am that all of the crew are safe."

The bespoke "super maxi" racing yacht was worth £3m to £4m and had been "going very fast, pushing for the record and crashing through the waves," Eddie Warden Owen, the chief executive of the Royal Ocean Racing Club, which runs the Fastnet race, said.

Earl Williams was at the helm of the yacht and told RTE radio how he heard a "big bang" just after rounding Fastnet rock.

"I looked up thinking it was something in the masts," he said. "But the boat was just capsizing. I realised it was the keel as the boat was going over. It started to turn upside down quite quickly and we didn't have a lot of time.

"We had people asleep aboard and people down below changing watches. Several of the crew from inside the boat were very lucky to get out in time."

The five who were swept away were wearing their full red sailing suits and bright yellow inflatable life-vests and were carrying global positioning system devices. Hypothermia was the greatest risk to their lives, Owen said.

A lifeboat from Baltimore, Co Cork, picked up the 16 who had scrambled on to the hull of Rambler 100, before a dive boat from another racing outfit, Team Phaedo, which was already in the area to take pictures of its yacht, joined the search for the missing five sailors and located them after picking up a signal from their tracking devices.

"We pulled aboard our dive vessel owner George David and his four crew members who had all been strapped together – cold but healthy, happy, safe and well," said an entry on the Team Phaedo Facebook page.

Touton was airlifted by helicopterto Kerry hospital for treatment for hypothermia and was said by Harvey to be "doing fine". The four others were taken to Baltimore where they were reunited with the rest of the crew.

Jerome Kirby described seeing the hull of the boat drifting away from him several hundred yards away.

"They didn't know we were in the water so nobody was looking for us," he said. "It was mad. We had people below. Luckily everybody had life vests on. If we didn't have life preservers on, it would have been a disaster. If they hadn't found us the other five could still be drifting out there."

"We were picked up with really capable people," said David after the rescue. "It is just amazing. We want to thank everybody ... Boats do tip over. We don't like it and its not a common event but it does happen from time to time.The keel fin has fractured. It has just snapped off. This is a piece of solid stainless steel. It is designed not to do what it did."